Mosquitoes are obligate vectors of pathogens that cause disease in humans and animals. Throughout their development, mosquitoes experience drastic changes in their environment. Eggs, larvae and pupae live in aquatic habitats where they feed on, or are in constant contact with, microorganisms. Adults, however, fly in terrestrial habitats where they feed on plant nectars and, in the case of females, on blood. At all points of their life cycle, mosquitoes are exposed to pathogens through breaches in their cuticle and through feeding. To resist or limit infection, mosquitoes mount powerful cellular and humoral innate immune responses against invading organisms. While a considerable amount of work has been done to understand mosquito immune responses in the adult life stage, little attention has been paid to how immune responses change throughout insect development. Likewise, little is known about how an infection during an immature stage affects the biology of the resultant adult. The proposed research aims to fill these knowledge gaps in mosquito biology, with the primary goals being to compare the antibacterial responses of larval and adult mosquitoes, and determine the effect of larval infection on adult life history traits. Specifically, we propose to compare the potency and composition of the antibacterial immune response of Anopheles gambiae larvae and adults, determine the effect of larval infection on a subsequent adult infection (including malaria), and determine the effect of larval infection on adult size, blood feeding propensity and fecundity. This project will yield significant insight into how mosquitoes interact with pathogens, and into how an immune challenge faced by a mosquito as an immature influences its ability to harbor disease-causing microbes. Data arising from this project could aid in the development of novel strategies to control the spread of mosquito-borne disease.